Substrates used in printed circuit boards generally comprise a polymeric coating over a base material. The substrates may be rigid or flexible. Typical base materials used in the industry are metallic or nonmetallic, such as steel or fiberglass mesh, respectively. In the case of flexible circuits, base materials are generally thin and the coating applied thereto is also thin so as to allow for flexibility of the substrate. Other substrates are also known in the art.
Typical coatings over the base materials for these printed circuits include phenolic resins, phenolic-rubber resin, ABS, various epoxy and rubber-modified epoxy resins. It has been found, especially with the use of rubber-modified epoxy resins, that in certain instances, especially with flexible circuit boards having rubber-modified epoxy coatings on the substrate, a problem exists which may be termed "solder sticking".
During the manufacture of a printed circuit assembly on a printed circuit substrate, solder is generally caused to be flowed over the substrate surface after the formation of the circuit pattern thereon. This solder is generally flowed over the surface during assembly of the printed circuit board with its associated electronic components by the wave soldering, solder dipping or drag soldering operations. All of the above soldering techniques are well known in the art. Ordinarily, the solder sticks only to the metallic circuit pattern previously applied to the substrate. However, occasionally, solder is found to stick to the substrate in areas between the circuit pattern or conductive pads. If the solder sticking is extensive or occurs between closely-spaced circuit lines or pads where a short circuit or leakage path may occur in the circuit, that particular circuit board may have to be rejected or reworked. This problem especially manifests itself in the newer circuit board technology wherein spacing between the lines or pads is often kept at a minimum.
There have been various efforts to control solder sticking involving many approaches including modifying the solder or soldering parameters, fluxes, and more recently a mechanical abrasion of the surface of the substrate. However, a more cost effective, and reliable method which does not alter or effect known solder or soldering parameters is desired.
We have made an unexpected discovery that exposure to short wavelength ultraviolet of printed circuits formed on partially uncured rubber-modified epoxy glass substrates as presently used in many flexible printed wiring boards, eliminates or substantially reduces solder sticking.